The invention relates generally to rotary drill bits for drilling boreholes in a subterranean formation. More particularly, the invention relates to systems and methods for leaching Polycrystalline Diamond (“PCD”) cutter elements to enhance their mechanical properties.
Oil and gas drilling operations often employ fixed cutter drill bits to drill through various rock formations in an effort to access hydrocarbon reserves below the ground. Fixed cutter drill bits employ a plurality of cutter elements that engage, scrape, and shear the earthen formation being drilled through. Such cutter elements are typically made of a layer or table of Polycrystalline Diamond (“PCD”) bonded to a cobalt cemented, tungsten carbide (WC) substrate.
To manufacture a PCD table for a cutter element and bond the table to the substrate, diamond powder is placed at the bottom of a first mold or can along with a catalyst. The substrate is then placed within the first mold on top of the diamond powder, a second mold or can is placed on top of the substrate, and a seal is formed between the first and second cans. This entire assembly is then subjected to high pressure and temperature conditions to form the PCD cutter element. In general, any Group VIII element (e.g., cobalt, nickel, or iron) can be used as the catalyst, however, in most cases, cobalt (Co) is employed. The high pressure and temperature conditions drive the catalyst into the interstitial spaces between the diamond grains and promotes intergrowth, thereby forming a solid PCD table suitable for use in a cutter element. The high pressure and temperature conditions also facilitate bonding between the newly formed PCD table and the substrate, thereby resulting in a fully formed PCD cutter element.
During drilling operations, cutter elements experience relatively high temperatures due, at least in part, to the general nature of the downhole environment and friction between the cutter elements and the formation. The thermal loads result in expansion of the material components of the cutter elements. Due to differences in the coefficients of thermal expansion between the catalyst and the diamond grains in the PCD table, at sufficiently high temperatures undesirable cracks may form in the PCD lattice structure of the table. Such cracks can lead to failure of the cutter element, reduced cutting efficiency, and reduced cutting effectiveness. Additionally, high thermal loads can also lead to the formation of materials such as, for example, carbon monoxide, carbon dioxide, or graphite within the PCD table itself, which can further reduce the effectiveness and strength of the cutter element. Accordingly, it is desirable to remove at least a portion of the catalyst from a PCD table after its formation to enhance cutter element durability over a broader range of operating temperatures.
A common approach to remove the catalyst from a PCD table is to leach the PCD table to remove some or substantially all of the interstitial catalyst from the PCD lattice structure, thereby transforming the PCD table into thermally stable polycrystalline diamond. Leaching typically involves placing the cutter element in a strong acid bath at an elevated temperature to expose the PCD table to the acid. Suitable acids for leaching include nitric acid, sulfuric acid, hydrofluoric acid, hydrochloric acid, and combinations thereof. Conventional leaching processes typically require large amounts of time in order to allow the leaching acid to remove the desired amount of catalyst from a given PCD table. In some cases, a PCD cutter element must remain within the leaching acid for up to three weeks in order to obtain the desired result. This relatively long time requirement reduces the flexibility available in manufacturing PCD cutter elements, thereby increasing the costs associated therewith.